A flip chip is generally defined as a chip mounted on a substrate by various interconnect materials and methods, which is characterized in that the active input-output area of the chip surface faces the substrate.
The classic flip chip technology is the IBM Controlled-Collapse Chip Connection (C4). This technology utilizes solder bumps deposited on solder wettable metal terminals on the chip and a matching footprint of solder wettable terminals on the substrate. The solder typically includes approximately 95% to 97% by weight of lead (Pb), with the remainder being made up by tin (Sn).
When mounting the chip to the substrate, the substrate is first prepared with solder flux and the chip is "flipped" onto the substrate with the solder bumps aligned with the substrate terminals. All of the solder joints are then formed simultaneously by reflowing (melting) the solder to achieve electrical connection. Residual flux is then removed using an appropriate solvent.
An underfill, or alternatively, an encapsulant material is then applied to the chip, and by capillary action, the space between the chip and the substrate is filled substantially completely by the underfill or encapsulant material. The chip and the substrate are then firmly bonded by curing the underfill or encapsulant material. The problem of large thermal mismatch between the chip and the substrate, which would otherwise cause solder fatigue problems, is reduced by an order of magnitude by the use of the underfill material or encapsulant between the flip chip and the substrate.